Brown adipose tissue (BAT) has garnered widespread attention recently due to its potential involvement in weight maintenance, blood lipid metabolism and glucose homeostasis. Although research in animals (mainly mice and rats) supports an inverse relationship between BAT and the development of metabolic syndrome, very little is known about the function of BAT in humans (hBAT). The reason for our limited understanding may be due in part to the lack of appropriate techniques for studying hBAT. The technique most commonly used to study hBAT is 18F-FDG PET, which has come under scrutiny because it only detects BAT that takes up glucose (not the preferred energy substrate of BAT). Therefore, 18F-FDG PET likely underreports the prevalence of hBAT. Our group has recently developed robust MRI-based techniques to study hBAT non- invasively, and without the use of radioactively labeled tracers. We will use these techniques to provide an accurate assessment of the prevalence and thermogenic activity of hBAT as well as to investigate the influence of physical activity, sex, age, body composition and diabetes status on hBAT. We will study hBAT in adults using two magnetic resonance imaging (MRI) techniques available on a 3 Tesla human MRI scanner. First, fat signal fraction (FSF) maps obtained from fat-water separated MRI can localize hBAT and estimate BAT volume and mass. Maps of FSF can distinguish BAT because it contains a higher proportion of water compared to white adipose tissue (WAT), while also containing a higher lipid fraction than lean tissue. The second MRI technique is a recently developed advanced fat-water thermometry technique that measures the absolute shift in the water peak resulting from a change in temperature. Lipid signals, which are not affected by temperature, serve as a built-in reference allowing the measurement of absolute temperature in tissues containing both water and lipid signals - a hallmark of brown adipose tissue. Subjects will also undergo indirect calorimetry. Mass and the thermogenic activity of hBAT will be assessed in each subject using two distinct experimental protocols using either cold-stimulation or a meal-challenge. In this study, we will pursue the following three Specific Aims: [1] Investigate the impact of physical activity and sex on BAT mass and activity in adult humans. [2] Investigate the impact of age and body composition on BAT mass and activity in adult humans. [3] Investigate the impact of diabetes on BAT mass and activity in adult humans. Impact: Reliable characterization of hBAT mass and activity in healthy and clinically relevant subjects is critical for the pursuit of longitudinal interventional studies by clinical investigatos to therapeutically enhance hBAT to counteract metabolic diseases.